Project Summary Direction selective ganglion cells in the mammalian retina are strongly activated by motion in their preferred direction, but are suppressed by motion in the opposite, or null, direction. They report the direction of motion to higher brain centers for further visual processing, and they contribute to the control of eye movements and, potentially, to conscious vision. Direction selectivity of these ganglion cells is attributed to multiple pre- and postsynaptic mechanisms. However, the implementation of these mechanisms at the synapse level is not fully understood. The goal of this proposal is to provide fundamental insights into the structure-function relationship of the synaptic circuitry that underlies direction selectivity. The proposed experiments will focus on synaptic inputs from the starburst amacrine cell, a critical interneuron that co-releases GABA and acetylcholine onto direction selective ganglion cells. We will first determine the properties of synaptic transmission and the functional wiring diagrams of the GABAergic and cholinergic circuits from starburst amacrine cells to direction selective ganglion cells, and will then identify the predominant synaptic mechanism underlying direction selectivity. We will take an innovative approach that combines genetic cell type-specific targeting, electrophysiology, fine resolution optogenetics and uncaging techniques to characterize and manipulate the synapse types of interest, and to correlate synaptic-level mechanisms with circuit function. This work will provide definitive answers to the outstanding questions that remain about the direction selective circuit. It will also contribute to the knowledge of the general principles that govern sensory processing. Moreover, this research will provide insight into the mechanism of chemical co-transmission in sensory systems and in higher brain structures.